1. Field of the Invention
This invention relates to a system for controlling the gear shift of an automatic transmission for a vehicle, and more particularly, to a system for controlling the gear shift of the automatic transmission in consideration of the conditions of a road on which the vehicle runs, the driving conditions of the vehicle, or a driver's mental state.
2. Description of the Related Art
An automatic transmission of a vehicle has a plurality of gear-shift stages to be selected for the forward drive of the vehicle. In the automatic transmission of this type, one gear-shift stage is automatically selected in accordance with the running conditions of the vehicle, that is, vehicle speed and engine throttle opening.
The automatic transmission is combined with a system for controlling the gear shift operation. This gear shift control system is previously provided with a plurality of up-shift lines and down-shift lines corresponding thereto. Based on the vehicle speed and the throttle opening, the up- and down-shift lines individually divide gear-shift stage regions on the low- and high-speed stages. These shift lines are used to select the gear-shift stage of the automatic transmission. In other words, whether the gear-shift stage of the automatic transmission should be shifted up or down is determined by the up- and down-shift lines.
More specifically, when the representative point of the driving conditions, that is, the shift point, which is determined by the actual vehicle speed and actual throttle opening, is moved across a certain down-shift line from a higher-speed stage region thereof to a lower-speed stage region, the gear shift control system delivers a shift command signal to the automatic transmission such that the current gear-shift stage should be shifted down to a gear-shift stage on the low-speed side defined by the down-shift line concerned. As a result, the gear-shift stage of the transmission is switched to a lower-speed stage.
When the shift point is moved across a certain up-shift line from a lower-speed stage thereof to a higher-speed stage region, the gear shift control system delivers a command signal for shift-up to the automatic transmission. As a result, the gear-shift stage of the transmission is switched to a higher-speed stage.
Conventionally, the up- and down-shift lines are determined by the vehicle speed and the throttle opening only. Depending on the running state of the vehicle, therefore, the gear-shift stage of the automatic transmission may be unexpectedly shifted or repeatedly shifted up and down.
Let us suppose a situation, for example, such that the accelerator pedal of the vehicle is released from the depression by a driver when the vehicle is on the point of entering a curve while ascending a slope, or when the driver sees an obstacle before his or her eyes, such as a wild plunge of a pedestrian or another vehicle, while the vehicle is running along the streets of a town. In such a situation, one of the up-shift lines may possibly be crossed by the shift point, which is determined by the actual vehicle speed and actual throttle opening, so that the automatic transmission is immediately shifted up to a gear-shift stage on a higher-speed side. Thereafter, when the driver depresses the accelerator pedal again, seeing that the vehicle has reached the end of the curve, or that the obstacle is removed, one of the down-shift lines is then crossed by the shift point, so that the automatic transmission is shifted down to a lower-speed stage. Thus, while the vehicle is running in the aforesaid state, the automatic transmission is repeatedly and alternately shifted up and down, although its shifting operation is practically unnecessary.
The above-described situation occurs while the vehicle is running at a relatively low speed. In some cases, however, the automatic transmission may repeat undesired shifting operations even while the vehicle is running at a relatively high speed. Let us suppose a situation, for example, such that the driver slowly and slightly deepens the depression of the accelerator pedal when the vehicle approaches a gentle ascending slope while running at a relatively high speed. Conventionally, in such a case, the additional depression of the accelerator pedal causes the shift point to cross one of the down-shift lines, so that the automatic transmission is shifted down from a higher-speed stage to a lower-speed stage. Thus, the gear-shift stage is shifted down. Since the upward slope is a gentle slope, moreover, the vehicle speed rapidly increases. When the depression of the accelerator pedal is restored to the original depth by the driver, in response to the increase of the vehicle speed, therefore, one of the up-shift lines is then crossed by the shift point, so that the gear-shift stage of the automatic transmission is shifted up again to the higher-speed stage. Thus, also in this situation, the gear-shift stage of the transmission inevitably continues to be alternately shifted between the lower- and higher-speed stages, so that the running stability of the vehicle is lowered.